X-ray tubes are applied to an X-ray image diagnosis, a nondestructive testing, etc. As the X-ray tubes, a stationary anode X-ray tube and a rotating-anode X-ray tube are present, and one of them is used in accordance with an intended use. The X-ray tubes each comprise an anode target, a cathode, and an envelope. A focus is formed in an anode target and emits X-rays when an electron beam collide the anode target.
The cathode comprises a filament coil and an electron convergence cup. The filament coil can emit electrons which produce a tube current. Between the anode target and the cathode, a high tube voltage of several tens of kilovolts to hundreds of kilovolts is applied. Thus, the electron convergence cup can serve as an electron lens; that is, it can cause an electron beam traveling toward the anode target to converge.
In general, the rotating-anode X-ray tube is applied to a medical diagnosis. Ordinarily, in the X-ray tube, the following two focal spots are provided: a large focal spot which has great dimensions and in which large tube current can be input; and a small focal spot which has small dimensions and in which small tube current is input, but a resolution is high. In addition, in a certain X-ray tube, three focal spots are provided. The dimensions of each of the focal spots depend on shapes of a filament coil and an electron convergence cup and a positional relationship between the filament coil and the electron convergence cup, and ordinarily, they are fixed. In the case where the large focal spot and the small focal spot are used, photographing conditions are determined after a spatial resolution and tube current (influencing a contrast and noise) are determined in accordance with an intended use in diagnosis, and either the large focal spot or the small focal spot is used properly.
However, in only two focal spots, photographing conditions are discontinuous, and there is a case where in an X-ray image diagnosis, a necessary image cannot be obtained. In particular, when photographing is continuously performed in an axial direction of a subject as in helical scanning in an X-ray CT apparatus, there is a case where variability of input with two focal spots which are discontinuous cannot ensure that continuity is maintained in image quality, and as a result, an accurate image diagnosis cannot be made. In view of this point, there is provided a method of changing dimensions of a focal spot by changing the voltages to be applied to a plurality of electrodes.
However, in such a focal-spot dimension changing method, a control and a structure are complicated, or it is necessary to exert a complicated control of adjustment of a tube current and dimensions of a focal spot. Furthermore, although a tube current which can be input is limited due to the dimensions of the focal spot, if the control of the current and that of the dimensions of the focal spot are exerted by different systems, there is a possibility that if those controls are in nonconformity to each other, an overcurrent will be caused, resulting in breakage of an X-ray tube.
Furthermore, in the case where the dimensions of the focal spot are changed, they are hard to control such that they are set to desired dimensions. For example, the variation of the length and width of the focal spot greatly differs from that of a bias voltage supplied to an electron convergence cup. Thus, it is difficult to adjust the dimensions of the focal spot and the amount of the tube current at the same time such that they are set to appropriate values. In view of this point, a technique is proposed in which an electrode which controls the length of a focal spot and an electrode which controls the width of the focal spot are prepared, and the focal spot is controlled to have desired dimensions.